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Full text of "Flora Indica Vol-I"

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214                                      FLORA   INDICA.
arid, but not quite Tibetan, Pinus Gerardiana being very
common.   Its flora is, however, scarcely known.
Tibet includes the mountain valleys of the Indus and Yaru
(or Brahmaputra), together with the whole axis of the Hima-
laya and the heads of many of the valleys which descend on
the Indian side, and which are situated beyond the mass of
snow throughout a great extent of the chain. Beyond the
Indus and Yaru are the southern slopes of the Kouenlun,
which according to our definition do not form a part of the
Himalaya, but of Tibet. Politically its boundary is an irre-
gulai one, accidental circumstances having regulated the line
of separation between the Indian and Tibetan states. Botani-
cally, the boundary of Tibet is best drawn at the place where
the climate becomes too arid to support such a vegetation
as flourishes at equal elevations on the Indian watershed,
and especially where there is a total absence of forests below
113,000 feet. The flanks of all the great Himalayan rivers,
when above 13,000 feet, are, owing to the elevation, devoid of
trees, whether the climate be humid or arid; but whcu their
course is oblique, as is the case with the Satlej and the Arzin,
there are no trees at far lower elevations than this/and a con-
siderable part of their upper coiwse is through a Tibetan cli-
mate. Thus, in the valley of the Satlej the climate is too dry
for trees at the junction of the Piti river, elevated 9000 feet ;
and the whole of Piti, as Well as the upper course of the Satlej
itself, forms part of Tibet. In the valleys of the Ganges and
Jumna, on the other hand, whose course is perpendicular to
the plains, trees ascend to 10,000 feet, and only the alpine
zone is arid ard hence belongs to the Tibetan Himalaya, in
contradistinction to " Himalaya interior."
Tibet may be divided into two parts, one to the westward
(the basins of the Indus and Satlej), the other to the eastward
(those of the Yaru and Arau, and perha >s r\ the Monas, Su-
huusiri, and other rivers). Tfrom the pos^ou of the llima-